Reference for Pesachim 221:11

הני בי תרי דמצעא להו אשה נדה אם תחלת נדתה היא הורגת א' מהן אם סוף נדתה היא מריבה עושה ביניהן מאי תקנתיה נפתח באל ונפסיק באל

Yet that is only where there is nought to fear of witchcraft; but where there is aught to fear of witchcraft, even if there are all these [safeguards], we still f as in the case of a certain man who rode on a ass and was wearing his shoes; his shoes shrank, and his feet withered. Our Rabbis taught: There are three who must not pass between [two men], nor may [others] pass between them, viz. : a dog, a palm tree, and a woman.

Jastrow